These are stocks that don't qualify to trade on any exchange, due to failure to file the financial reports, low trading volume, poor capitalization, etc.

Most investors should treat these extensions as warning signs. Usually these stocks are complete mysteries beyond speculation on message boards and press releases issued by the company. There is no verifiable information about them available. The stock price is hugely volatile and can swing up or down by huge amounts in the blink of an eye.

Stock brokers typically charge more to handle these types of trades.

Most people come into contact with these stocks when they're advertised by unsolicited e-mails or faxes. In these instances, usually it the sender of the email or fax has the goal of pumping up the hype to build artificial trading volume so he can sell his large block of shares at more than market value.

Most people who commit money to one of these stocks get burned. I suggest you stay away.